Controls for combined hydrostatic and multiple speed range transmission units with automatic speed control and braking functions

ABSTRACT

A drive train includes a hydrostatic transmission unit and a multiple speed range transmission unit arranged in series between a prime mover and a primary output shaft, variable displacement and torque transmitting capacity of the hydrostatic transmission unit being adjusted by hydraulic controls including a speed control valve for developing a differential pressure signal proportional to a desired rate of operation, a modulating valve for regulating the differential pressure signal corresponding to accelerating and decelerating operation of the transmission, a directional control valve for determining the direction of operation of the hydrostatic transmission unit, and a range selector valve for shifting the operating speed range of the multiple speed range transmission as the hydrostatic transmission approaches a limit of displacement, the displacement of the hydrostatic transmission being automatically reset in response to shifting of the multiple speed range transmission. 
     Operating speed limits are automatically established for the drive train by an override speed control valve which manually adjusts the differential pressure signal for reduced torque transmission through the drive train when the prime mover is operating below a minimum speed. A speed limiting control valve responds to operation of the prime mover above a maximum selected speed for automatically applying a brake within the drive train in order to supplement dynamic braking capacity of the transmission unit, the speed limiting control valve including means producing a feedback signal for resisting manual operation of the override speed control valve in proportion to engagement of the brake within the drive train.

This is a division of Ser. No. 609,637, filed Sept. 2, 1975, now U.S.Pat. No. 4,080,850.

BACKGROUND OF THE INVENTION

The present invention is directed toward numerous features within adrive train of a type having a transmission unit which is capable ofproviding a generally continuous positive coupling within the drivetrain. Such a transmission unit is preferably embodied within ahydrostatic unit including at least one hydraulic translating means suchas a pump or motor capable of variable displacement. Again, it ispreferable that both the pump and motor be capable of variabledisplacement.

The present invention is also directed toward broadening the operatingcapabilities of such a transmission unit by combining it in series witha multiple speed range transmission.

The present invention is particularly concerned with automaticallysynchronizing operation of the two transmission units to achieve asmooth transition of torque transmitting capacity and operating speedsfor the drive train.

The present invention is also concerned with providing automatic speedcontrols for a transmission unit of the type first noted above.Preferably, the means for synchronizing operation of the twotransmission units as well as means for accomplishing the speed controlfunctions referred to above are embodied in hydraulic controls asdescribed in greater detail below. However, it will be apparent from thefollowing description that the same or similar functions can be achievedthrough other control elements such as electronic control circuits.

Substantial efforts have been expended and are still being undertaken inan attempt to more effectively use the numerous advantages afforded byhydrostatic transmissions. Generally, hydrostatic transmissions presentspecial problems in control since displacement of both the pump andmotor must be varied in exact sequence in order to achieve efficientoperation and to provide suitable regulation over torque transmittingcapacity and operating speed of the drive train. For example, in such ahydrostatic transmission, the pump is commonly set at zero displacementwith the motor being at or near its maximum displacement when the drivetrain is in a neutral condition.

For acceleration of the vehicle, displacement of the pump may first bevaried toward a maximum value while the motor remains at is maximumdisplacement in order to develop maximum torque transmitting capacityfor initially accelerating the vehicle. After the pump reaches maximumdisplacement, displacement of the motor may be gradually reduced tofurther accelerate the vehicle.

Usually, as the motor approaches minimum displacement, the fulloperating range of the hydrostatic transmission is realized according tothe presently available prior art unless the transmission includesrelatively sophisticated developments such as multiple pumps forextending the torque transmitting capacity of the hydrostatictransmission. However, such solutions tend to make the transmissionsvery complex while even further increasing difficulties in properlysequencing operation of the variable displacement components therein.

Accordingly, it is desirable to provide a relatively simple and easilycontrolled means for expanding the torque transmitting capacity of ahydrostatic transmission unit in order to better adapt hydrostatictransmissions for use in a wide variety of vehicles. In particular,hydrostatic transmissions with expanded torque transmitting capacitywould be useful in material handling machines such as earth movingvehicles where a single prime mover is employed both to supply motivepower for the vehicle as well as to operate one or more implements whichmay also have substantial instantaneous power requirements relative tothe maximum output capability of the prime mover.

An improved hydrostatic transmission would be particularly useful insuch machinery for numerous reasons. For example, material handlingvehicles must be adapted both for transport operation at relative highspeeds as well as low speed, high torque operation of the vehicletogether with intermittent operation of its implements. At such times,the vehicle may be subjected to frequent changes of direction andcontinuous accelerating and/or declerating operation. A hydrostatictransmission unit is very suitable for such applications particularly ifautomatic controls are provided to maximize use of the available powerfrom the single prime mover.

A hydrostatic transmission could also be adapted for relieving theengine and increasing output torque during lug conditions by selectivelyand automatically reducing vehicle speed. In addition, a hydrostatictransmission would enable available power from a prime mover to be moreprecisely proportioned between what is required for motive power to thevehicle as well as supplying preferential power requirements of variousimplements mounted on or associated with the vehicle.

Examples of presently available hydrostatic transmissions for use insuch vehicles are set forth, for example, in U.S. Pat. No. 3,302,390 toChristenson and U.S. Pat. No. 3,477,225 to Cryder et al, the last notedpatent being assigned to the asignee of the present invention. TheChristenson patent discloses a transmission which is adapted foroperation of track-type vehicles whereas the present invention isparticularly intended for use with wheeled vehicles since it providesonly a single primary drive train. However, it will be apparent thatnumerous features of the present invention could also be used, forexample, with track-type vehicles including dual primary drive trains.

Other examples of prior art in the area of hydrostatic transmissionsinclude U.S. Pat. Nos. 3,187,509; 3,212,263; 3,236,049; 3,238,724;3,247,669; 3,273,344; 3,285,000; 3,324,797; 3,331,480 and 3,411,297.

SUMMARY AND OBJECTS OF THE PRESENT INVENTION

Accordingly, it is an object of the present invention to provide a drivetrain with improved controls for expanding the torque transmittingcapacity and operating speed range within the transmission and/orproviding for automatic operation of numerous functions within thetransmission to improve and facilitate its operation.

Another object of the present invention is to provide a multiple speedrange transmission unit in combination with a transmission unitproviding a positive drive coupling during both acceleration anddeceleration of the drive train while also being adjustable forinfinitely variable torque transmission capacity and speed of operation.A transmission unit exhibiting a positive drive coupling is peferablyembodied as, but not limited to, a hydrostatic transmission.

The present invention provides controls for selectively regulatingtorque transmitting capacity of the positive drive coupling transmissionunit while automatically shifting the operating range of the multiplespeed range transmission when the first transmission unit approaches anoperating limit. Preferably, the first transmission unit is reset atsubstantially the same time that the multiple speed range transmissionis shifted in order to permit its continued response to a control signalfor varying torque transmitting capacity within the new speed range.

It is a further object of the present invention to provide controls fora transmission unit providing a positive drive coupling during bothaccelerating and decelerating operation wherein a modulated signal isproduced to closely regulate both accelerating and deceleratingoperation of the transmission. Preferably, a single modulating unit isemployed to regulate the control signal representative of bothaccelerating and decelerating conditions within the drive train.

It is also an object of the invention to provide such controls for atransmission of the type noted immediately above wherein separate meansare employed to regulate the rate of deceleration for the drive trainwhen the direction of operation for the drive train is being reversed.

It is another object of the invention to provide a control assembly fora hydrostatic transmission unit wherein a variable differential pressuresignal is produced in proportion to a desired rate of operation for thedrive train, the differential pressure signal regulating numerous valvecomponents which control operation of the hydrostatic transmission unitas well as for synchronizing its operation with that of a multiple speedrange transmission.

Yet another object of the present invention is to provide a controlassembly for a transmission unit of the type exhibiting a positive drivecoupling during accelerating and decelerating conditions, a manualoverride control element being operable to selectively decelerateoperation of the drive train independent from normal signal controlsregulating its accelerating and decelerating operation.

A still further object of the invention is to provide a speed limitingcontrol means for automatically applying a brake within the drive trainwhen its prime mover is being driven at excessive rates of speed.

A corresponding object of the invention is to provide the speed limitingcontrol means in combination with the manual override means referred toimmediately above together with feedback means for resisting operationof the manual override means in proportion to engagement of the brakewithin the drive train.

Still another object of the invention is to provide a method forsynchronizing operation of a hydrostatic transmission unit and amultiple speed range transmission unit arranged in series within a drivetrain, the method being characterized in that the multiple speed rangetransmission unit is shifted between speed ranges and, almostsimultaneously, the hydrostatic transmission unit is reset in order topermit continued displacement variation in response to a control signal.

Another object of the invention is to provide a method for regulatingoperating speed of a transmission unit providing a positive couplingduring both accelerating and decelerating operating conditions of thedrive train wherein means producing a signal for normally establishingaccelerating and decelerating rates of operation for the transmissionmay be overridden by separate means to selectively decelerate the drivetrain and also wherein a brake may be applied within the drive train inresponse to operation of a prime mover at excessive speeds in order tosupplement dynamic braking capacity available within the transmissionunit.

Additional objects and advantages of the invention are made apparent inthe following description having reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially schematic representation of a drive trainincluding a hydrostatic transmission unit and a multiple speed rangetransmission unit together with a hydraulic circuit for controllingoperation of the two transmission units as well as for supplyingnecessary fluid to the hydrostatic transmission unit.

FIG. 2 is a schematic representation of a control group of elementswithin the hydraulic circuit for regulating operation of the twotransmission units.

FIG. 3 illustrates the composite arrangement of FIGS. 4-12 to provide amore detailed representation, with parts in section, of the controlassembly of FIG. 2.

FIG. 4, within the composition figure, includes a safety reset valve.

FIG. 5, within the composition figure, includes a speed control valveand a modulating orifice valve.

FIG. 6, within the composition figure, includes a directional valve anda fluid accumulator.

FIG. 7, within the composition figure, includes a range selector valve.

FIG. 8, within the composition figure, includes an override speedcontrol valve assembly.

FIG. 9, within the composition figure, includes an underspeed controlvalve.

FIG. 10, within the composition figure, includes an overspeed controlvalve and a brake pressure control valve.

FIG. 11, within the composition figure, includes a portion of thedisplacement actuator for the hydrostatic motor together with a pilotmotor for operating the motor actuator.

FIG. 12, within the composition figure, includes a pilot motor andassociated actuator for the hydrostatic pump unit.

FIG. 13 is a graphical representation illustrating accelerating and/ordecelerating response of the combined transmission units of the presentinvention to a control signal.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In view of the relative complexity of the present invention and thedrive train embodying the invention, the following description ispresented under the following divisions:

(1) The drive train and associated hydraulic supply and controls of FIG.1.

(2) A summary of the hydraulic control assembly illustrated in FIG. 2and composite FIGS. 4-12.

(3) Detailed description of the control assembly having reference tocomposite FIGS. 4-12.

(4) Description of the preferred mode of operation.

Generally, it will be noted that the present invention is described withreference to a drive train including a hydrostatic transmission unit anda multiple speed range transmission unit under the regulation of acontrol assembly comprising a number of hydraulic valve componentsforming a hydraulic control circuit. However, it is emphasized againthat numerous variations are possible within the scope of the presentinvention.

Further, it will be clearly apparent from the following description thatthe various control valve assembly components described below forregulating operation of the two transmission units, as illustrated inFIGS. 2-12, may readily be replaced by other control elements capable ofperforming the same or similar functions. In this connection, it will beparticularly obvious that the novel control functions of the presentinvention may also be accomplished, for example, by means of anelectronic control circuit.

(1) The drive train and associated hydraulic supply and controls of FIG.1.

Referring now to FIG. 1, a drive train 20 is schematically representedas including a prime mover or engine 22 with a hydrostatic transmissionunit 24 and a multiple speed range transmission unit 26 being arrangedin series between the prime mover 22 and a primary output shaft 28suitable for providing motive power in a vehicle (not shown). The primemover 22 is also directly coupled in driving relation with a pump 30representative of an implement (not shown) having intermittent powerrequirements which are substantial relative to available power from theprime mover 22.

If the drive train 20 is employed for example, in a loader vehicle, theimplement pump 30 could be employed to operate a bucket or othermaterial handling means arranged upon lift arms of the loader vehicle(not shown). The actual identity of the implement is not of importanceto the present invention except to note that power requirements of theimplement together with motive power requirements for the vehicle may incombination exceed the available power from the prime mover 22.Accordingly, it is desirable to efficiently employ power from the primemover 22 and to closely regulate operation of the drive train so thatboth motive and implement power may be available when required.

The hydrostatic transmission unit 24 is of a type including at least onevariable displacement translating device such as the pump indicated at32 and the motor indicated at 34. The hydrostatic pump and motor areinterconnected by means of a hydrostatic loop comprising lines ormanifolds 36 and 38 which are suitably adapted for high pressureoperation of the hydrostatic transmission.

Construction details of such a pump and motor within a hydrostatictransmission unit may be seen, for example, in U.S. Pat. No. 3,381,472,which is assigned to the assignee of the present invention. For purposesof this disclosure, it is sufficient to understand that the pump andmotor units 32 and 34 are respectively rotated about their trunnionmountings 40 in order to selectively change or vary their displacement.The pump 32 is rotated by means of a hydraulic servo actuator 42 whilethe motor 34 is rotated by operation of another servo actuator 44.Within the preferred embodiment of the present invention, the motoractuator 44 is preferably arranged to have its piston 46 fixed with itshousing or cylinder 48 being coupled for movement with the motor 34 inorder to facilitate operation of a pilot means 50 in a manner describedin greater detail below.

The hydrostatic transmission 24 is illustrated in a neutral controlcondition with the pump 32 being positioned for minimum or zerodisplacement and the motor 34 being arranged at or near a position ofmaximum displacement.

Because of the closed loop mode of operation between the hydrostaticpump 32 and motor 34, little fluid is lost from the transmission so thatonly a limited amount of make-up fluid need be added to the hydrostaticcircuit. Accordingly, a conventional relief and replenishing valve group52 is provided in communication with the hydrostatic lines 36 and 38 inorder to assure an ample fluid supply and to maintain a suitabletemperature range for fluid within the hydrostatic transmissioncomponents. The relief and replenishing valve group is adapted foroperation at high pressure while being capable of removing or addingfluid to either of the hydrostatic lines 36 and 38 depending upon theirrelative pressurization.

Fluid under pressure which is supplied to the transmission units as wellas being employed to regulate their operation in a manner describedbelow, is delivered by means of a pump 54 which is also driven by theprime mover 22. The pump 54 is of a proportional type supplying outputflow in proportion to operating speed of the prime mover 22. The pump 54draws fluid from a reservoir 56 and directs it through a filter 58toward a venturi orifice unit 60 including conventional thermalcompensating means 62. Pressure taps 64, 66 and 68 are in respectivecommunication with the venturi inlet conduit 70, the venturi throat 72,and the outlet conduit 74 for control purposes, described in greaterdetail below. Fluid from the outlet conduit 74 is delivered to a flowcontrol valve 76 having a spring-loaded spool 78 forming a restrictiveorifice 80 which communicates the conduit 74 with another conduit 82.Operation of the spring-loaded spool 78 provides generally constantvolume flow, of for example, 7.5 gallons per minute, so the conduit 82.As the spool 78 is shifted rightwardly by increased flow from theconduit 74, excess fluid is communicated into another conduit 84 whichis in communication with a variable flow relief valve 86. The variablerelief valve 86 maintains a selected supply pressure within the conduit84 for purposes described below while communicating excess fluid intoone of a pair of conduits 88 and 90 which are in respectivecommunication with the relief and replenishing valve group 52.

Excess or high temperature fluid from the relief and replenishing valvegroup 52 is also communicated through the conduit 90 to apressure-responsive cooler by-pass and relief valve 92 which selectivelydirects the fluid either through a cooler 94 or a by-pass conduit 96 toa jet pump 98. The jet pump 98 also draws fluid from the multiple speedrange transmission 26 through a conduit 100 with fluid passing throughthe jet pump 98 and the supplemental cooler 94 being reduced to asuitable temperature range before it is returned to the reservoir 56.

The various portions of the hydraulic supply and control circuit of FIG.1, as described immediately above, combine to permit the separate pump54 to supply the various fluid requirements for operation and regulationof the drive train. A more detailed description of those components isavailable in U.S. Pat. No. 3,877,224, issued Apr. 15, 1975, that patentbeing assigned to the assignee of the present invention.

The hydraulic circuit of FIG. 1 also includes a control valve assembly102 which is in respective communication with the conduits describedabove and indicated respectively at 64, 66, 68, 82, 84, and 88. Thevalve assembly 102 is effective to communicate fluid signal throughconduits 210 and 228 for operating the pump's servo 246 and actuator 42.The valve assembly 102 is also operable to develop fluid signals inadditional conduits 108 and 110 which are in communication with thepilot motor or the valve 50 which in turn operates the actuator 44 forthe hydrostatic motor 34. The various components within the controlvalve assembly 102 together with its mode of operation are describedbelow with reference to FIG. 2 and composite FIGS. 4-12.

(2) Summary of the hydraulic control assembly illustrated in FIG. 2 andin composite FIGS. 4-12.

Before proceeding with a detailed description of the construction andoperation for the various valve components within the control valveassembly, the various functional purposes of the valve components may besummarized as follows. The relative location of the valve componentswithin the control valve assembly may be best seen in FIG. 2 while thedetailed construction of each valve component is illustrated within thecomposite FIGS. 4-12. The venturi unit 60, the flow control valve 76 andthe hydrostatic motor actuator 44 which were described above inconnection with FIG. 1, while not strictly a part of the control valveassembly, are nevertheless included within FIG. 2 and composite FIGS.4-12 in order to better indicate the path of fluid flow into and throughthe control valve assembly 102.

As an initial element within the control valve assembly 102, a speedcontrol valve 112 is manually operable by an operator to indicate adesired rate of operation for the drive train of FIG. 1 or its vehicle.The speed control valve accomplishes its function by generating asignal, preferably a differential pressure signal within a pair ofconduits, as described in greater detail below, which varies forregulating operation of the hydrostatic transmission unit and themultiple speed range transmission unit after first being modified andcontrolled by other valve components within the control valve assembly.

A modulating valve 114 acts upon the variable signal generated by thespeed control valve in order to regulate both the rate of increase forthe signal, corresponding to acceleration of the drive train, as well asthe rate of decrease for the signal which corresponds to deceleration ofthe drive train. The modulating valve 114 accomplishes both of thesepurposes by means of a common modulating orifice valve which will bedescribed in greater detail below.

The modulated variable signal generated by the speed control valve 112and adjusted by the modulating valve 114 is applied to the pilot controlvalves for the hydrostatic motor and pump actuators through adirectional valve 116. The directional valve 116 preferably performsthree basic functions. Initially, it establishes the direction ofoperation for the drive train by regulating the manner in which themodulated signal is communicated to the pilot control valves. Secondly,the directional valve includes means (described below) for closelyregulating the sequence in which displacement variation of thehydrostatic pump and motor is to take place. Thirdly, the directionalvalve 116 establishes a selected rate of deceleration when the directionof operation is changed, that rate of deceleration being independentfrom the normal rate of deceleration established by the modulating valve114.

A range selector valve 118 operates in response to the modulatedvariable signal from the speed control valve for automaticallyestablishing one of the multiple speed ranges within the multiple speedrange transmission unit 26 (See FIG. 1). Operation of the range selectorvalve for causing a shift between speed ranges is regulated by anaccumulator 120 which also receives the modulated signal from the speedcontrol valve 112 and the modulating valve 114.

The accumulator 120 also serves to absorb undesirable pressure surges inthe modulated variable signal from the speed control valve 112 and themodulating valve 114, particularly during directional changes initiatedby the directional valve 116 and speed range shifts initiated by therange selector valve 118 and accompanied by response of the pilotcontrol valve 50 to reset displacement of the hydrostatic motor.

In connection with operation of the range selector valve 118, it is alsoimportant to note that the pilot control valve 50 for the motor actuatorincludes means responsive to shifting of the range selector valve forresetting displacement of the hydrostatic motor in order to permit itscontinued response to the modulated signal for further acceleration ordeceleration in the new speed range. Preferably, resetting of thehydrostatic motor is accomplished without affecting the modulated signalfrom the speed control valve 112 and the modulating valve 114 in orderto permit smoother operation of the hydrostatic transmission unit inconjunction with the multiple speed range transmission unit (See FIG.1).

A safety control valve 122 preferably operates in conjunction with thespeed control valve 112 and prevents development of a variable signalafter start-up of the prime mover 22 (See FIG. 1) until the manualcontrol element of the speed control valve 112 is first returned to aneutral setting. Thus, the safety control valve 122 assures that thecontrol valve assembly 102 is properly conditioned to initiateaccelerating operation of the transmission after start up.

A number of components within the control valve assembly 102 perform agenerally common function of regulating operating speed of the primemover 22 (See FIG. 1), particularly below or above a preselectedoperating speed range. Initially, an underspeed control valve 124functions to adjust the modulated variable signal from the speed controlvalve 112 and the modulating valve 114 when the operating speed of theprime mover 22 is below a preselected level. Thus, when the transmissionis operating under heavy load conditions, its torque load is reduced inorder to permit the operating speed of the prime mover to return to asatisfactory range.

An override speed control valve 126 acts upon the modulated variablesignal from the speed control valve 112 and the modulating valve 114 issubstantially the same manner as the underspeed control valve but undermanual control in order to enable an operator to selectively reduce theoperating speed of the drive train. The particular manner in which thisfunction is accomplished does not require resetting of the speed controlvalve so that a preselected speed setting may be maintained within thespeed control valve. Additionally, the override speed control valve 126permits a feedback function discussed immediately below in connectionwith a speed limiting control valve 128.

The speed limiting control valve 128 performs the basic function ofgenerating a signal for the purpose of applying a brake within the drivetrain whenever the operating speed of the prime mover 22 exceeds apreselected maximum value. Thus, when the prime mover 22 tends to bedriven in operation through the drive train, for example, when a vehicleis travelling downhill, the operating speed is automatically limited atgenerally the setting established by the speed control valve 112.

The speed limiting control valve 128 performs an additional function inconjunction with the manually operable override speed control valve 126.Normally, overspeeding of the prime mover 22 occurs when an operator isattempting to reduce operating speed of the drive train or vehiclethrough manipulation of the override speed control valve 126.Accordingly, the speed limiting control valve is designed to generate afeedback signal which resists manual operation of the override controlvalve 126 in order to indicate to the operator the degree of engagementfor the brakes within the drive train. Thus, the override speed controlvalve 126 may be freely adjusted by its manually controlled element toemploy dynamic braking capacity within the hydrostatic transmission unitfor reducing speed of the drive train. However, when the speed limitingcontrol valve initiates engagement of the supplemental brakes within thedrive train, the degree of engagement for the supplemental brakes isthus signalled to the operator so that he is aware of their use indecelerating the drive train. The feedback signal generated by the speedlimiting control valve is adjusted in response to engagement pressure ofthe brake as well as operating speed of the prime mover and outputoperating speed of the drive train in order to provide a true indicationto the operator as to the amount of supplemental braking being providedby the brakes.

Finally, the brake pressure control valve 129 functions in response tothe brake engagement signal from the speed limiting control valve 128 inorder to selectively pressurize or engage the brake within the drivetrain. Preferably, the brake pressure control valve 129 is adapted tocommunicate actuating pressure for the brake from one of the hydrostaticmanifolds, whichever is at a higher pressure.

It may be seen from the above summary that the underspeed control valve124, the override speed control valve 126 and the speed limiting controlvalve 128 function in combination to automatically regulate operatingspeeds for the drive train. Operation of the underspeed control valve124 is relatively conventional. However, the valve components 126 and128 novelly permit the employment of dynamic braking capacity of thehydrostatic transmission to the fullest extent possible, thereaftercomputing the amount of supplemental braking capacity required tomaintain operation of the prime mover within acceptable limits. Thiscomputing function extends further to generation of the feedback signaldiscussed above in order to signal the operator as to the amount ofsupplemental braking capacity being employed within the drive train.These functions for the valve components 126 and 128 may readily beaccomplished by means other than the hydraulic valves illustrated anddescribed. The use of an electronic control circuit is particularlysuggested for this purpose.

(3) Detailed description of the control valve assembly 102.

A complete representation, including cross-sectional views of thevarious valve components in the control valve assembly 102, is providedby composite FIGS. 4-12. Fluid is supplied at a constant volume flowrate from the venturi 60 through the flow control valve 76 (FIG. 1) tothe control valve assembly 102 through the conduit 82.

Pressurized fluid necessary for operation of the hydraulic actuatorswhich vary displacement of the pump 32 and motor 34 and which engagesthe clutches of the multiple speed range transmission unit enters thecontrol valve assembly through the conduit 84.

Fluid in conduit 82 is communicated to the speed control valve 112. Thevalve 112 produces a differential pressure signal for actuating orregulating other components in the control assembly 102. When a manualcontrol spool 130 is in a neutral position within the valve 112, fluidpasses freely into another signal conduit 132 for passage through thevalve 126 and 124 before returning to the relief and replenishingcircuit, as illustrated in FIG. 1.

The valve 112 is illustrated in greater detail in composite FIG. 5.Fluid enters the valve 112 from the conduit 82 and flows through anannular groove 134 and a pair of metering slots 136 on the spool 130, anannular recess 137 in the valve 112 and then into the conduit 132. Whenthe spool 130 is shifted rightwardly, the slots 136 restrict flowthereacross so that pressure rises in the conduit 82.

A passage 138 in the valve 112 communicates the conduit 82 with conduit140 through a check valve 142 for communication to the valves 126 and124 for a purpose described below.

When the spool 130 of the speed control valve 112 is shifted leftwardly,low pressure signal fluid from the conduit 132 is directed to a conduit144 in communication with the end of a spool 146 (see FIG. 4)reciprocably located in the safety control valve 122. The reset functionof the valve 122 is initiated when the spool 146 is thus shiftedupwardly against a spring 148. Pressure from the conduit 88 in a branchconduit 150 and passage 152 is thereby communicated across a groove 154in the spool 146 to a conduit 156 which releases the parking brake 158and delivers pressurized fluid to the pilot stage of relief andreplenishing valve 52 via a conduit 157, (also see FIG. 1). Thus, theconduit 157 serves to vent the relief and replenishing valve group atthe same time that the parking brake 158 is applied. In addition,communication between conduits 160 and 162 is blocked for a purposedescribed below.

As the spool 130 of the valve 112 is shifted rightwardly toward itsmaximum speed control position, it blocks conduit 144 which remainspressurized by fluid from the groove 154 flowing through an orifice 164in order to maintain the spool 146 shifted upwardly against the spring148. If a malfunction should occur within the control system, causing alowering of pressure in the conduit 144, the spring 148 would then movethe spool 146 downwardly toward its vent position while fluid in theconduit 144 generally escapes past groove 166 into a drain conduit 168to provide a timed delay before the parking brake is allowed to engage.

The time delay feature is provided to prevent minor pressurefluctuations from affecting operation of the safety control valve 122. Acheck valve 170 prevents excess pressure from escaping the conduit 144into the conduit 150.

When the spool 130 of the valve 112 is moved completely to the left, theconduit 144 is communicated to a drain conduit 172 which enables thespool 146 to be moved downwardly by the spring 148, thus venting thetransmission and applying the parking brake.

When fluid pressure enters the conduit 82, it also flows into themodulating valve 114 through a branch conduit 174 while lower pressuresignal fluid in the conduit 132 is directed to that valve through aconduit 176 by operation of the valve 112 as described above.

The pressure of fluid entering the modulating valve 114 is initiallyadjusted by a pressure regulating reducing valve 178 and communicated toa passageway 180. Fluid under pressure in the passage 180 iscommunicated into a chamber 182 in a modulating orifice valve spool 184and another passage 186. The passage 186 communicates with a springchamber 188 for a second pressure regulating reducing valve 190 which isthereby responsive to pressure established by the first pressureregulating valve 178 in the passage 180. Fluid from the chamber 182passes through modulating orifices 192 into a passage 194 and thenacross the pressure regulating valve 190 into a high pressure signalconduit 196.

A branch conduit 198 directs high pressure signal fluid from the conduit196 into a chamber 200 located on the left end of the modulating spool184. At the same time, low pressure signal fluid from the conduit 176biases the modulating spool 184 so that it is thus responsive to thesame differential pressure applied to the pilot operated actuators 42and 44 for the hydrostatic pump and motor, as will be described ingreater detail below, the differential pressure thus being a function ofoutput speed of the drive train.

The valve 114 modulates or adjusts pressure in the conduit 196 byregulating the flow rate into and out of the spring-loaded pump pilotcylinder 211 and accumulator 120. Spring characteristics within theaccumulator 120 and pilot cylinder 211 are selected so that, as pressurein the conduit 196 increases, the pump pilot cylinder 211 moves first,the accumulator 120 moving second in unison with the motor actuator 44.Thus, increased pressure in the conduit 196 results in an increase inoutput speed for the drive train.

In summary, the steady-state pressure level in the conduit 196 isdetermined by the instant setting for the valve 112 while the valve 114modulates the transition or rate of pressure change in the conduit 196from one level to another.

Because of the opposed arrangement of the pressure regulating valves 178and 190, they operate in conjunction with the single modulating valvespool 184 to regulate fluid flow in either direction between conduits174 and 196, thus establishing the rates of both acceleration anddeceleration for the hydrostatic transmission. The pressure regulatingvalves 178 and 190 operate in conjunction with the modulating valvespool 184 to establish a fixed pressure drop regardless of pressurefluctuations caused primarily by manual operation of the spool controlvalve 112 so that the instantaneous rate of acceleration or decelerationis the same at any given speed of operation.

The novel construction of the modulating valve assembly 114 also permitsthree separate and independent adjustments--corresponding toacceleration rate, deceleration rate and the characteristic rate changefor the modulating valve 184--for example, by adjusting or changing thebiasing force acting on each of the regulating valves 178 and 190 andthe modulating valve spool 184 by means of their respective springs 202,204, and 206.

The conduit 196 communicates high pressure signal fluid to a directionalcontrol valve 116 which determines the direction of travel for the drivetrain by varying the direction in which displacement of the hydrostaticpump 32 occurs.

Low pressure signal fluid is also communicated to the valve 116 throughthe conduit 132.

The position of a manually adjustable spool 208 determines the directionof travel for the drive train. For forward operation, the spool 208 isshifted to the position illustrated in the composite FIG. 6. Highpressure signal fluid in the conduit 196 is then communicated throughconduit 210 to one end of a pilot control cylinder 211 for regulatingthe pump actuator 42 (also see FIG. 1). The cylinder 211 includes apiston 213 acted upon by opposed centering springs 215 and 216.

With the spool 146 of the safety control valve 122 shifted upwardly, theconduit 162 is blocked from the conduit 160 to permit pressurization ofthe conduits 210 and 162 (see FIG. 4).

The directional valve 116 (see FIG. 6) also contains a sequencing spool212 which shifts to direct pressurized fluid to the pilot valve 50 forthe hydrostatic motor 34. The motor 34 is, of course, not reversiblelike the hydrostatic pump 32. When the spool 208 shifts (to the positionshown) to pressurize the conduit 210, it also communicates high pressuresignal fluid through a passage 214 to the sequencing valve spool 212.Fluid flow through a passage 218 in the spool 212 shifts the spoolupwardly against the centering spring assembly 220. The passage 214 isthus communicated with a conduit 222 which leads to the pilot controlvalve 50 for the hydrostatic motor, the accumulator 120 and the rangeselector valve 118 (see FIG. 7).

When the spool 208 is shifted downwardly, corresponding to reverseoperation, pressurized fluid in the conduit 196 is transmitted throughan axial passage 226 in the spool to a conduit 228. Pressurization ofthe conduit 228 transmits fluid to the opposite end of the pump pilotcylinder 211 in order to shift the pump 32 in the opposite direction.However, since the conduit 228 is in communication with a passage 230leading to the sequencing valve spool 212, high pressure signal fluid isalso communicated to a chamber 232 at the top of the spool 212 by meansof a passage 234. The spool 212 is thus shifted so that the passage 230communicates with the conduit 222 for the motor pilot valve 50.

Regardless of the direction in which the spool 208 is shifted, pressurein either passage 214 or 230 is also directed to a passage 236 and achamber 238 to act upon a spool 240 containing variable orifices 242.

During the deceleration portion of a forward-reverse shift which isinitiated by the directional spool 208, the spool 212 maintains itsposition until pressure acting on either end of the spool diminishes.Thus, fluid escaping from the pump pilot cylinder in conduit 210 mustflow through the variable orifices 242 thence to the low pilot pressureconduit 132. The variable orifices 242 are controlled by vehicle speed,as represented by pressure in the passage 236, in order to provide aprogrammed rate of deceleration only during directional shifts. Thesequencing spool 212 subsequently shifts back to its centered positionso that subsequent acceleration, in reverse, is again regulated by themodulating valve assembly 114.

As indicated above, conduit 210 communicates pressurized fluid to thepump pilot cylinder 211, which acts upon a servo actuator valve 246 (seeFIG. 12) so that a servo-coupled valve spool 244 is shifted to directfluid from the conduit 84 to the actuator cylinder 247. The cylinder 247is part of the actuator 42 for the hydrostatic pump 32.

As pressure increases in the conduit 210, the piston 213 movesrightwardly against the spring 215 and fluid pressure from the conduit228 which is connected to the low pressure pilot signal conduit 132. Asthe piston 213 is shifted rightwardly, the spool 244 also movesrightwardly. Fluid pressure is directed from the conduit 24 through thepassage 217 to the chamber 219 in order to move the piston 224rightwardly. The cylinder 211 is thus moved leftwardly in order toadjust pump displacement. The piston 213 is thereby shifted leftwardlyin order to block the conduit 24 from the passage 217 and limit leftwardmovement of the cylinder 211. The servo actuator functions in a similarmanner in reverse. For example, pressure in the conduit 228 is thenincreased in order to shift the piston 213 against the spring 216 andthe relatively low signal pressure from the conduit 210.

As the high pressure signal fluid is increased by the speed controlvalve 112, displacement of the pump is first varied. As the pumpapproaches maximum displacement, further pressure increases arecommunicated across the sequencing spool 212 to shift a spool 248 of themotor pilot valve 50 (see FIG. 11) which in turn directs fluid from theconduit 84 to the motor actuating cylinder in order to changedisplacement of the motor, as described in greater detail below.

The range selector valve 118 (see FIG. 7) functions automatically toshift speed ranges as the motor approaches minimum displacement ormaximum speed. Low pressure signal fluid in the conduit 132 iscommunicated to a chamber 250 of the accumulator 120 (see FIG. 6). Thatchamber is also communicated to a conduit 252 by means of an orifice254. The conduit 252 leads to a chamber 256 at the left end of a spool258, as seen in FIG. 7, reciprocably located in the range selector valve118. An orifice 260 in a drain passage 262 permits pressurization of thechamber 256 which shifts the spool 258 rightwardly to the positionshown.

Once the spool 258 is moved rightwardly, pressure is communicated fromthe conduit 222 into a conduit 264 across a groove 266 formed on thespool 258. Also, with the spool in that position, fluid under pressurefrom the conduit 84 is directed to a conduit 268 across a groove 270 onthe spool 258 in order to direct fluid under pressure to engage a lowrange clutch 272 of the multiple speed range transmission 26 (also seeFIG. 1). Supply fluid from the conduit 84 is also directed to a chamber274 and acts between a slug 276 and the spool 258 in order to assist inmaintaining 258 in its rightwardly shifted position.

As the speed control valve spool is moved further toward the right,increasing signal fluid pressure develops in the conduit 222 which iscommunicated to the accumulator 120 and the motor pilot valve 50. As thepressure in the conduit 222 increases, it shifts a piston 278 upwardlyin the accumulator 120 against a spring 279, FIG. 6. The piston movesupwardly until the chamber 250 is communicated with a conduit 280 byorifices 282 in order to pressurize a chamber 284 at the right end ofthe spool 258. Pressure in the chamber 284 acts upon the spool 258 andshifts it to the left. This occurs because the piston 278 in theaccumulator 120 moves upwardly to block the line 252, thereby allowingfluid pressure in the chamber 256 to escape through the drain line 262.

When the spool 258 shifts leftwardly, the conduit 268 is communicated toa drain conduit 286 across the groove 270 while a conduit 288 iscommunicated with the fluid supply conduit 84 across a groove 290 on thespool 258. Thus, the low range clutch 272 is deactuated and a high rangeclutch 292 is simultaneously engaged. Also, fluid pressure from theconduit 84 is communicated to a passage 294 in the spool 258 in order towork against a slug 296 and assist in holding the spool 258 in itsleftwardly shifted position. The chamber 274 is simultaneously vented toa drain line 298.

Accordingly, to summarize accelerating operation, the multiple speedrange transmission is initially in a low speed range and, as speedincreases, the increasing differential pressure in the signal conduitsfirst shifts the pump pilot valve to increase the pump 32 from zerotoward maximum displacement. After the pump reaches maximumdisplacement, further increases in the differential pressure begin toshift the motor 34 from maximum displacement toward a minimumdisplacement condition.

In order to further increase operating speed, increasing differentialpressure shifts the multiple speed range transmission unit from low tohigh range operation as described above, while at the same time shiftingthe motor displacement back toward maximum displacement as describedbelow. After the high range clutch 292 is engaged, further increases indifferential pressure again cause the motor to shift from maximum towardminimum displacement.

Rapid shifting of the motor from the minimum to maximum displacement isaccomplished as follows: The motor pilot valve 50 and hydraulic actuator44 are shown in detail on FIG. 11. As stated, high pressure signal fluidis communicated through the conduits 222 and 264, leading to the leftend of the pilot valve 50. Low pressure signal fluid is directed to theright end of the pilot valve through the conduits 132 and 110. Theconduits 222 and 108 connect with a chamber 300, formed in a piston 302by means of a passage 304 in the pilot valve, a passage 308 formed inthe spool 248 and a passage 306 formed in the piston 302. The conduit264 is in similar communication with a chamber 310 formed in the spool248. Pressure in the chamber 300 acts against the left end of a plunger312. The plunger 312 also has its right end in contact with the spool248. The combination of forces developed in the chambers 300 and 310 iscounteracted on the opposite end of the spool 248 by low pressure signalfluid communicated from the conduit 110 and acting in a chamber 314formed between a piston 316 and the spool 248. Rightward travel of thepiston 316 is limited by a fixed pin 318. The spool 248 is also urgedleftwardly by a spring pack 320 located at the right end of the spool248 (see FIG. 2).

Increasing pressure in the chambers 300 and 310 eventually causes thespool 248 to move rightwardly so that actuator supply fluid from theconduit 84 is directed to a chamber 322 in the motor actuator 44. Themotor actuator piston rod 46 moves to the left (see FIG. 11) and changesdisplacement of the motor 34. The spring 320 is thus compressed tobalance a higher pressure in chambers 300 and 310. When the forces onthe spool 248 are balanced, the spool returns to its center position,shown in FIG. 11, and the motor ceases to change position. Thisincremental operation occurs for every incremental increase of signalfluid pressure, until the hydrostatic motor is shifted to a minimumdisplacement setting, in order to provide maximum output speed in agiven speed range.

At the same time increasing fluid signal pressure in the conduit 222 iscommunicated into the bottom of the accumulator 120 as the hydrostaticmotor 34 is shifted toward minimum displacement, the passages 282 in theaccumulator piston 278 begin to enter into communication with theconduit 280 through an orifice 324. Thus, the accumulator providesprecise timing for directing low pressure signal fluid into the conduit280 in order to shift the range selector valve spool 258 leftwardly fordisengagement of the low speed clutch 272 and engagement of the highspeed clutch 292. This function was also described above. However, atthe same time, the conduit 264 which previously contained high pressuresignal fluid is communicated with the low pressure signal fluid conduit132 by means of the groove 266 formed on the right end of the spool 258.The resulting reduction of pressure in the conduit 264 is alsoimmediately reflected within the chamber 310 in the pilot valve 50 sothat substantially constant pressure within the chamber 314 shifts thespool 248 leftwardly. Thus, the pilot valve 50 directs actuating fluidpressure from the conduit 84 into a chamber 326 at the head end of themotor actuator 44 in order to rapidly shift the motor back toward aposition of maximum displacement.

After the motor is rapidly shifted back toward its position of maximumdisplacement as described above, the pilot valve 50 continues to respondto further incremental increases in the differential signal pressure toagain shift the motor toward a condition of minimum displacement inorder to provide acceleration in the high speed range.

It is particularly important to note that the motor 34 is shifted backto its position of maximum displacement by means independent of pressurein the signal conduits 222 and 132. Rather, the motor is merely shiftedby effectively reducing pressurization in the chamber 310 of the motorpilot valve 50. Pressure escaping from the chamber 310 may be absorbedwithin the accumulator 120 so that it does not affect any other portionof the control valve assembly. Consequently, a shift from low to highoperating speed range does not result in uneven operation for the drivetrain since it is not necessary to generate a conventional underspeedsignal when the drive train is shifted into its high operating speedrange.

The remaining portion of the description for the control valve assembly102 is directed toward the valve components 124, 126, 128 and 129 whichfunction to automatically regulate the differential signal pressurewithin the control assembly 102 and thus operating speed limits for thedrive train.

The underspeed control valve 124 corresponds generally in function andmode of operation with a similar underspeed control valve (80) describedin U.S. Pat. No. 3,477,225, assigned to the assignee of the presentinvention. Accordingly, the construction and mode of operation for theunderspeed control valve 124 is only briefly described below. Highpressure fluid from the conduit 140 is in communication with a passage328 in the valve 124. It is also important to note that the conduit 140is in communication with the high pressure fluid signal conduit 222 bymeans of a check valve 330 (see composite FIG. 5 and FIG. 2). Generally,the underspeed control valve functions in response to operation of theprime mover 22 below a predetermined minimum value in order tocommunicate the conduit 140 and accordingly the high pressure signalconduit 222 with the low pressure signal conduit 132 and its downstreamconduit 88. The underspeed control valve performs this function inresponse to a fluid signal received from the venturi unit 60 through theventuri signal conduits 64 and 66. Note that the pressure differentialbetween those two conduits is representative of operating speed for theprime mover 22.

The underspeed control valve 124 includes a metering spool 332 forregulating fluid communication from the conduit 140 and passage 328 intoanother branched passage 334 in communication with the low pressureconduits 132 and 88. Fluid pressure from the venturi signal conduit 64is communicated through a passage 336 to act against the left end of thespool 332. Similarly, fluid from the venturi signal conduit 66 iscommunicated through a passage 336 to act against the left end of thespool 332. Similarly, fluid from the venturi signal conduit 66 iscommunicated through a passage 338 into a spring chamber at the rightend of the spool 332.

During relatively high speed operation of the prime mover, pressure inthe signal conduit 66 is relatively low so that the spool 332 remainsshifted toward the right, in the position illustrated in FIG. 9, bypressure from the signal conduit 64. However, as operating speed of theprime mover decreases below a predetermined minimum value, pressure inthe conduit 66 increases in relation to pressure within the conduit 64.Accordingly, fluid pressure in the chamber 340 combines with force ofthe spring 342 to shift the spool 332 leftwardly and relieve some of thefluid pressure from the conduit 140 and accordingly, from the highpressure signal conduit 222, thus allowing the pump or motor toeffectively reduce the output speed of the vehicle. This, of course,reduces torque loading on the prime mover 22. When operating speed ofthe prime mover recovers, pressure in the venturi signal conduit 66diminished, permitting the spool 332 to be shifted rightwardly so that adifferential pressure may again be developed within the conduit 222.

The override speed control valve 126 also receives high pressure fluidthrough the conduit 140 from the speed control valve 112. Low pressuresignal fluid is also communicated across the valve 126 by means of theconduit 132 as noted above.

The purpose of the override speed control valve 126 is to permit anoperator to selectively reduce operating speed of the drive train (seeFIG. 1) without necessarily adjusting or resetting a speed control valve112. This feature is of course of particular value in material handlingvehicles where an operator is busy manipulating implements as well asregulating the operating speed and the direction of the vehicle.Accordingly, the override speed control valve 126 permits him toestablish a desired operating speed by means of the speed control valve112 and to maintain that setting while intermittently reducing operatingspeed through use of the valve 126.

The operator may selectively reduce operating speed by manually shiftinga control rod 344 leftwardly to compress a spring 346 (see FIG. 8).Compression of the spring 346 acts through an adapter 348 and a piston350 against a pivoted lever 352. Resulting movement of the lever 352urges a spool 354 rightwardly against its spring 356 in order tocommunicate relatively high pressure fluid from the conduit 140 to thelow pressure conduit 132. As noted above, this permits high pressuresignal fluid from the conduit 222 to escape through the check valve 330,thereby reducing the differential pressure in the signal conduits 132and 222 in order to reduce operating speed of the drive train.

During operation of the override speed control valve 126 in the mannerdescribed above, the prime mover is often subjected to overspeedconditions while attempting to provide necessary dynamic braking throughthe hydrostatic transmission. This condition occurs for example when thevehicle is operating at a high rate of speed or when the vehicle istraveling downhill. At such times, the dynamic braking capacity of theprime mover and hydrostatic transmission may be insufficient todecelerate the vehicle at the desired rate. Accordingly, the presentinvention contemplates supplemental brakes which are automaticallyoperated to supply additional braking capacity in response to operatingconditions within the drive train.

In order to prevent such overspeeding, the speed limiting control valve128 (see FIG. 10) is selectively operable in a manner describedimmediately below to case variable engagement of brake 360 within thedrive train 20. The brake 360 is schematically represented on FIG. 10 inconjunction with the brake pressure control valve 129. The speedlimiting control valve receives a low pressure signal from the venturisignal conduit 66 through branched conduits 362 and 364. The valve 128also receives an inlet pressure signal from the venturi unit 60 by meansof the signal conduit 64. It is again important to note that the signalreceived from the signal conduits 64 and 66, in combination, provide anindication of operating speed for the prime mover 22, as describedabove. An override spool 366, normally urged into the positionillustrated by a spring 368, is acted upon by fluid pressure from eachof the conduits 64 and 66. Under normal operating conditions where theprime mover is within an acceptable speed range, relative pressure inthe signal conduits 64 and 66 is not sufficient to urge the spool 366upwardly. However, when operating speed of the prime mover increasesabove a predetermined maximum valve, as described above, relativepressure in the signal conduit 64 increases and acts against the spring368 to shift the spool 366 upwardly in order to permit variable fluidcommunication from the actuator conduit 84 into the brake supply conduit370. Thus, conduit 370 is normally pressurized, except for a conditionof overspeed of the prime mover. Under this condition, the brakepressure control valve 129 is responsive to variable pressure in theconduit 370 for correspondingly engaging the brake 360.

Returning again to the speed limiting control valve 128, the conduit 370has a branch conduit 372 for communicating the actuating brake fluidpressure from the conduit 370 through the branch conduit 372 into apassage 374 in the override speed control valve 126 (also see compositeFIG. 8). Brake actuating pressure in the passage 374 acts through apiston 376 which is thus urged against the lever 352 in parallel withthe manual control rod 344. Rightward movement of the piston 376 islimited by a pin 378 only for the purpose of preventing rapidoscillation of the piston 376 in response to pressure fluctuations inthe passage 374.

The fluid signal communicated to the override speed control valve 126through the conduit 372 is only a portion of a feedback signal generatedby the speed limiting control valve 128 to provide an indication to theoperator of the mount of engagement for the drive train brake 360. Anadditional fluid pressure signal is communicated from the speed limitingcontrol valve to the override speed control valve through a conduit 380.

In order to develop the feedback signal within the conduit 380, fluidfrom the signal conduit 68 is communicated to a passage 382 of the speedlimiting control valve 128. A first regulating spool 384, including aset of variable orifices 386 and a single orifice 387, functions insubstantially the same manner as the spool 366 in response todifferential pressure in the conduits 64 and 66. For example, when theprime mover 22 is operating within an acceptable speed range, pressurein the venturi signal conduit 64 is not sufficient relative to pressurein the conduit 66 to urge the spool 384 upwardly against its spring 389.

However, as operating speed of the prime mover increases above apredetermined maximum level, relative pressure is increased within thesignal conduit 64 which serves to shift the spool 384 upwardly so thatthe orifices 386 begin to communicate a variable feedback signal intothe passage 388 and the conduit 380, as further described below.

In addition, the orifice 387 provides selective communication betweenthe passage 382 and the conduit 222.

The passage 382 is always pressurized to a relatively higher degree thanthe conduit 222. The purpose of the orifice 387 is to increase pressurein the conduit 222 during overspeed conditions of the prime mover. Forexample, if the operator sets the speed control valve 112 at a selectedlevel, pressure in the conduit 222 will tend to approach a correspondingsetting during normal operation. However, pressurized fluid communicatedfrom the passage 382 through the orifice 387 also increases the pressurelevel in the conduit 222. This additional pressurization in the conduit222 serves as an artificial signal corresponding to a further increasein output speed of the hydrostatic transmission in order to reduce theoverspeed condition of the prime mover.

The feedback signal in the passage 388 is also acted upon by a secondregulating spool 390 which is responsive to high pressure signal fluidfrom the conduit 222 and low pressure signal fluid from the conduit 132.The differential pressure between these two conduits is of courseproportional to output speed of the drive train so that the spool 390 isoperable for further adjusting the feedback signal in the passage 388 inproportion to operating speed of the drive train or vehicle.

In operation, the spool 390 is normally urged upwardly by its spring392. Low pressure signal fluid from the conduit 132 acts upon the spool390 in conjunction with its spring 392. High signal pressure from theconduit 222 acts upon the other end of the spool 390 in a chamber 394.Accordingly, a pressure differential between the conduits 222 and 132 issufficient to shift the spool 390 downwardly against its spring in orderto provide variable communication for the passage 388 with a drainpassage 396 through a slot 391.

Thus, the feedback signal supplied into the conduit 380 is proportionalboth to operating speed of the prime mover as well as output speed ofthe drive train which, in combination with the signal in the branchconduit 372, provides a true indication as to the amount of brakingeffort that is instantly provided by both the drive train brake 360 andthe prime mover.

The feedback signal from the conduit 390 is communicated to a passage398 in the override speed control valve 126 and a chamber 400 in orderto act upon the spool 354 through a slug 402. Thus, by means of thelever 352 and the spool 354, the feedback signal within the chamber 400serves to resist manipulation of the control rod 344 in proportion toengagement of the brake 360. Any hydraulic delays or fluctuations in thesystem are compensated for by a check valve 404 and a restrictiveorifice 406 arranged in parallel between the passage 398 and the chamber400.

From the immediately preceding description, it may be seen that theoverride speed control valve 126 and the speed limiting control valve128 automatically function in combination to compute the amount ofbraking capacity required in addition to the dynamic capacity of thedrive train in order to maintain operating speed of the prime moverwithin acceptable limits. The speed limiting control valve furtherfunctions to automatically apply the supplemental brake within the drivetrain while delivering a feedback signal to the override speed controlvalve as an indication to the operator of the amount of engagement forthe drive train brake.

The brake pressure control valve 129 merely functions in response to afluid signal from the speed limiting control valve 128 in the conduit370 in order to proportionally apply the brake 360. It is noted that thebrake 360 is of a type being normally disengaged. A regulating spool 408is normally shifted downwardly by pressure in conduit 370 acting againstspring 410. Thus, any pressure in passage 412 is blocked from the brake360. As the signal from the conduit 370 decreases, due to overspeed ofthe prime mover, the spring 410 shifts the spool 408 upwardly to allowcommunication between the passage 412 and the brake 360. Any pressurecommunicated to the brake 360 also acts upon the spool 408 in oppositionto the spring 410 by means of an interconnecting passage 414. Fluid fromthe passage 414 acts upon a slug 416 which is accordingly urgeddownwardly against the spool 408. The brake 360 is then disengaged byvariable communication with a drain passage 418.

The brake pressure control valve 129 also includes a shuttle valve 410which is automatically shifted in order to supply actuating fluid to thepassage 412 from one of a pair of conduits 422 and 424. Preferably, theconduits 422 and 424 are in respective communication with the manifolds36 and 38 for the hydrostatic transmission (also see FIG. 1). Sinceeither of the manifolds may be filled with high pressure fluid dependingupon the direction of operation for the transmission, the shuttle valvespool 420 serves to assure that the relatively high pressure side of thehydrostatic transmission is in communication with the passage 412 inorder to assure adequate pressure for engaging the brake 360.

(4) Detailed description of the preferred mode of operation.

It is believed that the mode of operation for the control valve assembly102 is clearly set forth in the above description. However, the mode ofoperation for the various valve components within the control assembly102 is briefly summarized below in order to assure a betterunderstanding of their combined operation.

Initially, the speed control valve 112 is manually operable to develop arelatively high pressure in the conduit 174 relative to the low pressuresignal conduit 132. The signal from the conduit 174 is modulated duringeither accelerating or decelerating operation of the transmission toprovide a variable high pressure signal in the conduit 196.

The directional valve 116 responds to the differential pressure in theconduits 196 and 132 for performing three functions. Initially, thedirectional valve determines the direction of operation for the pumpactuator 42 (see FIG. 1) in order to determine forward or reverseoperation of the drive train. Secondly, the directional valveestablishes the sequence in which displacement of the hydrostatic pump32 and motor 34 takes place. Finally, the directional valve includes athird valve component for modulating a decreasing differential pressurein the conduits 196 and 132 in order to regulate deceleration of thehydrostatic transmission only during a direction change. As thetransmission passes through a neutral condition, the modulating valve114 thereafter functions to again regulate the rate of acceleration forthe transmission.

The range selector valve 118 functions to automatically shift a multiplespeed range transmission unit 26 (FIG. 1) into a different speed rangeas the hydrostatic transmission approaches a select limit ofdisplacement. Concurrently, the pilot control valve 50 for thehydrostatic motor actuator 48 responds to the differential pressuresignal in conduits 196 or 222 and the low pressure signal conduit 132.The function of the range selector valve 118 and the automatic responseof the pilot control valve 50 is timed by operation of the accumulator120.

Before operation of the transmission can be initiated, however, it isfirst necessary that the safety control valve 122 be positioned todisengage a parking brake 158 (see FIG. 4) and also to blockcommunication between the conduits 160 and 162 so that differentialpressurization may be developed within the low pressure signal conduit132 and the high pressure signal conduit 222 or 196. The safety controlvalve 122, of course, provides these functions when the speed controlvalve 112 is first returned to a neutral position so that thetransmission units may thereafter be accelerated in proper sequence.

As noted above, the underspeed control valve 124 functions in agenerally conventional fashion to decrease the differential signal inthe conduits 132 and 196 when the prime mover is operating beneath apredetermined minimum speed level.

The override speed control valve 126 provides a means for selectivelyaccomplishing the same purpose in order to decelerate the drive train atany time desired by the operator.

Any corresponding overspeed conditions of the prime mover are sensed bythe speed limiting control valve 128 which accordingly causes engagementof a brake 360 within the drive train in order to supplement dynamicbraking capacity of the hydrostatic transmission unit. The speedlimiting control valve 128 further communicates a feedback signal to theoverride speed control valve 126 which is proportional to actualengagement of the brake 360, actual operating speed of the prime moverand actual output speed of the drive train in order to provide a trueindication to the operator as to the combined degree of braking from thebrake 360, and the prime mover through the drive train.

The graph of FIG. 13 also clarifies the manner in which the combinedhydrostatic and multiple speed range transmission units function incombination. Referring now to FIG. 13, the curve indicated at 502represents the angle of displacement for the pump 32 (regardless ofoperating direction) while the curve 504 represents the angle ofdisplacement for the motor 34 (also see FIG. 1). At zero speed of thetransmission, for example, during start up conditions with the speedcontrol valve 112 being in neutral, the pump is at minimum displacementwhile the motor is at maximum displacement. As the operating speed ofthe drive train is increased through manipulation of the speed controlvalve 112, the pump 32 is first shifted from minimum toward maximumdisplacemeht. As it reaches maximum displacement as indicated at 506,the motor 34 begins to experience displacement variation from itsmaximum condition toward a minimum. As displacement of the motor 34approaches a condition of minimum displacement indicated at 508 on thegraph, the range selector valve 118 (see FIG. 2) automatically shiftsthe multiple speed range transmission unit into a different speed range.Simultaneously, displacement of the motor is shifted to reset backtoward its condition of maximum displacement. Resetting of the motor 34occurs between points 508 and 510 on the graph. Thereafter, the motoragain continues to experience gradually decreasing displacement in orderto provide acceleration within the higher speed range settingestablished by the range selector valve.

The above procedure graphically represented in FIG. 13 may be performedin opposite relation for decelerating operation of the drive train.Thus, the graph of FIG. 13 indicates that the shift between speedranges, indicated between the points 508 and 510, may take place at anypoint selected along the curve in order to establish the most desirabletorque transmitting characteristics within the drive train. This featureis particularly facilitated by the automatic and simultaneousconditioning of the multiple speed range transmission and thehydrostatic motor in response to a single differential signal.

What is claimed is:
 1. A drive train for coupling a prime mover with anoutput shaft, comprisinga transmission unit arranged between the primemover and output shaft and providing a positive coupling during bothaccelerating and decelerating operation of the drive train, thetransmission unit being operable for producing infinitely variabletorque transmission and operating speeds, control means for producing avariable signal to which the transmission unit is responsive forestablishing the rate of torque transmission and the operating speed ofthe transmission unit, an underspeed control means which is responsiveto operation of the prime mover at a speed below a selected minimumvalue for adjusting the variable signal in order to reduce torqueloading of the drive train, an override speed control means beingmanually operable for also adjusting the variable signal in order toselectively decrease operating speed of the drive train withoutnecessarily adjusting the control means, and a speed limiting controlmeans which is responsive to operation of the prime mover at a speedabove a selected maximum value for applying brake means within the drivetrain in order to supplement dynamic braking capacity of thetransmission unit.
 2. The drive train of claim 1 further comprisingmeans responsive to engagement of the brake means within the drive trainby the speed limiting control means in order to develop a feedbacksignal proportional to engagement of the brake means which resistsmanual operation of the override speed control means.
 3. The drive trainof claim 2 wherein the feedback means are incorporated within the speedlimiting control means, the feedback means including means responsive toengagement of the brake means, means responsive to operating speed ofthe primary output shaft and additional means responsive to operatingspeed of the prime mover for jointly adjusting the feedback signalcommunicated to the override speed control means.
 4. A control assemblyfor a drive train including a prime mover, a primary output shaft and atransmission unit arranged in series between the prime mover and theoutput shaft to provide a positive coupling during both accelerating anddecelerating operation of the drive train, the transmission unit beingoperable for delivering infinitely variable torque transmission andoperating speed of the output shaft, comprisinga control means includingmanually operable means for producing a signal representative of desiredspeed for the output shaft, the signal being variable by the manualcontrol means in order to both accelerate and decelerate the outputshaft, and a speed limiting control means for selectively engaging brakemeans within the drive train in response to operation of the prime moverat speeds exceeding a selected maximum value in order to supplementdynamic braking capacity of the transmission unit, the speed limitingcontrol means thereby computing the need for supplemental brakingcapacity in response to overspeeds developed in the prime mover.
 5. Thecontrol assembly of claim 4 further comprising an override speed controlmeans being manually operable to adjust the signal from the controlmeans in order to decrease operating speed of the transmission unit, andmeans for communicating a feedback signal to the override speed controlmeans in proportion to engagement of the supplemental brakes forresisting manual operation of the override speed control means.
 6. Thecontrol assembly of claim 4 wherein the feedback means is incorporatedwithin the speed limiting control means, the speed limiting controlmeans comprising means responsive to rotating speed of the output shaftand additional means responsive to operating speed of the prime moverfor adjusting the feedback signal communicated to the speed limitingcontrol means.
 7. In a control assembly for a drive train including aprime mover, a primary output shaft and a transmission unit arranged inseries between the prime mover and the output shaft to provide apositive coupling during both accelerating and decelerating operation ofthe drive train, the transmission unit being operable for producinginfinitely variable torque transmission and operating speed of theoutput shaft, comprisinghydraulically operable pilot means forregulating operation of the transmission unit to selectively vary itstorque transmission capacity and operating speed of the output shaft, asource of fluid under pressure, a control valve assembly for selectivelycommunicating fluid from the source to the pilot means, the controlvalve assembly including manually operable means for varying fluidpressure communicated to the pilot means in proportion to the desiredoperating speed for the primary output shaft, the fluid pressure beingvariable by the manually control means in order to establish the rate ofboth acceleration and deceleration within the drive train, and a speedlimiting control valve which is in communication with the control valveassembly and a brake means within the drive train, the speed limitingcontrol valve including means responsive to operating speed of the primemover in excess of a selected maximum value for causing engagment of thebrake means within the drive train in order to supplement dynamicbraking capacity of the transmission unit in direct proportion tobraking requirements for the drive train as determined by overspeedconditions in the prime mover.
 8. The control assembly of claim 7further comprising an override speed control valve including manuallyoperable means for adjusting fluid pressure communicated from thecontrol valve to the pilot means in order to decrease operating speed ofthe output shaft and means for communicating a feedback signal to theoverride speed control valve in proportion to engagement of the brakemeans, the feedback signal tending to resist manual operation of theoverride speed control valve.
 9. The control assembly of claim 8 whereinthe feedback means is incorporated within the speed limiting controlvalve, the speed limiting control valve comprising a first valveeffectively responsive to engagement of the brake means for adjustingthe feedback signal communicated to the speed limiting control valve, asecond valve effectively responsive to the rotating speed of the outputshaft for further adjusting the feedback signal and a third valveeffectively responsive to operating speed of the prime mover for alsoadjusting the feedback signal.
 10. In a method of regulating operatingspeed within a drive train including a prime mover and a transmissionunit arranged in series between the prime mover and a primary outputshaft to provide a positive coupling during both accelerating anddecelerating operation of the drive train, the transmission unit havingan infinitely variable torque transmitting capacity with correspondingoperating speed, the steps comprisingproducing a modulated signal towhich the transmission unit is responsive for causing accelerating anddecelerating operation of the drive train, selectively adjusting themodulated signal by means of a manual override control element inresponse to operation of the prime mover at speeds below a selectedminimum for causing deceleration of the drive train, and applying abrake within the drive train in response to operation of the prime moverat speeds above a selected maximum value in order to supplement dynamicbraking capacity of the transmission unit in direct proportion tobraking requirement for the drive train as determined by overspeedconditions in the prime mover.
 11. The method of claim 10 furthercomprising the steps of producing a feedback signal,adjusting saidsignal in response to engagement of the brake within the drive train,further adjusting the feedback signal in response to operating speed ofthe output shaft, further adjusting the feedback signal in response tooperating speed of the prime mover, and applying the adjusted feedbacksignal to the manual override control element for resisting itsoperation as an indication of the amount of brake engagement within thedrive train.